To transmit a signal on a channel of a communications network, the protocols for access to a standard medium (for example a medium conforming to the IEEE 802.11 DCF standard), require that a radio transceiver is switched on at all times and always ready to receive the signal. This “ready to receive” mode consumes a lot of energy. If there is no transmission on the channel that energy is wasted in passive listening. This problem is clearly apparent in ad hoc low-traffic sensor networks, in which the channel is unoccupied most of the time.
To solve this problem there are preamble sampling transmission methods that reduce the passive listening overhead.
FIG. 8 shows diagrammatically the variation over time 125 in the transmission between a sender node 101a and a receiver node 101b of a signal 111 comprising a preamble 115 and a data frame 113. The preamble sampling protocol entails for a node listening to the radio channel intermittently.
The time axis 125 of the receiver node 101b is divided into brief periods represented by small squares 129 during which the radio is switched on (i.e. during which the receiver node 101b is active). These brief listening periods 129 are spaced by long periods 127 during which the radio is switched off (i.e. during which the receiver node 101b is inactive). The time axis 125 of the sender node 101a shows the sending of the signal 111 comprising the data frame 113 preceded by the preamble 115.
Accordingly, in protocols of this kind, a node has its radio switched off most of the time in order to reduce passive listening and therefore save energy.
Generally speaking, the node must periodically verify whether there is a signal (or a message) intended for it, for example once every Tw seconds (period 127). For this purpose, the node is activated during a short period 129 to listen to the channel.
If the node finds that the channel is free, it is deactivated again (its radio is switched off). In contrast, if it detects the presence of a preamble (known bit pattern) on the channel, it remains active (period 117) to receive the data frame 113 that follows the preamble 115. After receiving the data frame 113 the node is deactivated again. The preamble 115 is a long frame that contains repeating known bit patterns. This enables an active node to recognize a pattern and therefore to tell that it is a preamble 115.
Moreover, when a node wishes to send a data frame it listens to the channel first. If it finds that the channel is busy, it continues to listen until the channel is released. In contrast, if it finds that the channel is free, it sends a preamble before sending a data frame. To ensure that all potential receiver nodes are active during sending of the preamble, so that they do not miss the data that follows the preamble, the duration of the preamble must be at least equal to Tw.
A known method of reducing the size of the preamble is proposed by Enz C C., El-Hoiydi A., Decotignie J., and Peiris V. in their paper “WiseNET: An Ultra Low-Power Wireless Sensor Network Solution” (IEEE Computer, 37(8): 62-70, August 2004). That document describes an algorithm which uses small size preamble sampling to reduce the cost of point-to-point (unicast) messages. That reduces the transmission and reception time and therefore saves energy.
However, that method does not reduce the cost of multipoint (broadcast) communication, where listening to the preamble is of no utility.
Moreover, when a node wishes to send and finds the channel busy, it continues to listen until the channel is released, in order to send its frame. The node cannot tell when the channel will next be released. Such unnecessary listening consumes energy.
Moreover, in that method, a node can receive redundant data frames because it has no means for verifying whether it has already received the same data frames, and thus a lot of energy is wasted to no purpose.